1. Field of the Invention
The present invention relates to an electric power steering apparatus. More particularly, the present invention relates to an electric power steering apparatus capable of controlling a current thereof and a method of controlling the current, which can compensate for a current of a motor so as to increase auxiliary steering power of a vehicle in response to an increase of friction caused by contraction or an increase of viscosity at a low temperature, such as in the winter season.
2. Description of the Prior Art
A steering apparatus of a vehicle has generally adopted a hydraulic power steering apparatus using hydraulic power of a hydraulic power pump. However, an electric power steering apparatus using a motor has been gradually and widely used since 1990.
In the existing hydraulic power steering apparatus, a hydraulic pump, serving as a power source for supplying auxiliary steering power, is driven by an engine, to always consume energy regardless of the rotation of a steering wheel. However, in the electric power steering apparatus, when a steering torque is generated by the rotation of the steering wheel, the motor supplies auxiliary steering power proportional to the generated steering torque. Therefore, the electric power steering apparatus can improve the energy efficiency more than hydraulic power steering apparatus.
FIG. 1 is a view illustrating the electric power steering apparatus according to a prior art. As illustrated in FIG. 1, the electric power steering apparatus generally includes a steering system 100 extending from a steering wheel 101 to wheels 108 in both sides and an auxiliary power mechanism 120 for supplying auxiliary steering power to the steering system 100.
The steering system 100 includes a steering shaft 102, of which an upper end is connected to the steering wheel 101 to rotate together with the steering wheel 101 and a lower end is connected to a pinion shaft 104 through a pair of universal joints 103. Further, the pinion shaft 104 is connected to a rack bar 109 through a rack-pinion mechanism part 105 and both ends of the rack bar 109 are connected to the wheels 108 of a vehicle through a tie rod 106 and a knuckle arm 107.
The rack-pinion mechanism part 105 is provided with a pinion gear 111, which is formed at a lower end of the pinion shaft 104, engaged with a rack gear 112 formed at one side of an outer peripheral surface of the rack bar 109. The rotational movement of the pinion shaft 104 is changed to a linear movement by the rack-pinion mechanism part 105. Therefore, when a driver operates the steering wheel 101, the pinion shaft 104 rotates, the rack bar 109 linearly moves in a shaft direction according to the rotation of the pinion shaft 104, and the linear movement of the rack bar 109 steers the wheels 108 through the tie rod 106 and the knuckle arm 107.
The auxiliary power mechanism 120 includes a torque sensor 121 for sensing steering torque applied to the steering wheel 101 by the driver and outputting an electric signal proportional to the sensed steering torque, an Electric Control Unit (ECU) 123 for generating a control signal based on the electric signal transferred from the torque sensor 121, a motor 130 for generating auxiliary steering power based on the control signal transferred from the ECU 123, and an electric power apparatus 140 for transferring auxiliary power generated in the motor 130 to the rack bar 109 through a gear, a belt, etc.
Therefore, in the electric power steering apparatus as described above, the steering torque generated by the rotation of the steering wheel 101 is transferred to the rack bar 109 through the rack-pinion mechanism part 105, and the auxiliary steering power generated in the motor 130 according to the generated steering torque is transferred to the rack bar 109 through the electric power apparatus 140.
Contrary to FIG. 1, the electric power steering apparatus can be constructed in such a manner as to transfer the auxiliary steering power generated in the motor 130 to the steering shaft 102 or the pinion shaft 104 through the electric power apparatus 140. That is, the steering torque generated in the steering system 100 is combined with the auxiliary steering power generated in the motor 130, to move the rack bar 109 in the shaft direction.
However, in the aforementioned electric power steering apparatus, when the user starts and initially steers a car which has been left for a long time in a cold weather or at a low temperature, such as in the winter season, the larger steering power than that of a normal condition is required. Such a phenomenon is caused by the contraction of the elements of the electric power apparatus or the increase of viscosity of a lubricating fluid, such as guris, at a lower temperature.
Therefore, a compensation means for controlling the current of the motor that generates the power supporting the steering power according to the temperature has been demanded.